This invention relates to a retroreflector. In the usual practice, a series of light-reflecting units is disposed in an array which extends over an appreciable area of a retroreflector to provide a light-reflecting action. In prior light-reflecting arrays, the practice has been to make all of the light-reflecting units substantially of the same size and same geometric configuration. Patentability, in some instances, has been bottomed on the particular, uniform configuration used. Retroreflectors having light-reflecting units of substantially the same size and configuration are typified by the following U.S. Pat. Nos. 1,671,086 to Stimson; 3,332,327 to Heenan; 3,684,348 to Rowland; 3,758,191 to Hedgewick; and 3,923,378 to Heenan.
An array having light-reflecting units of substantially the same size and geometric configuration normally has maximum reflectivity in one particular direction, and the performance by the light-reflecting array falls off rapidly at other directions. However, there are many applications where appreciable retroreflection in only one particular direction is not desirable and, to the contrary, it becomes highly desirable to have reflectivity over a wide range of entrance angles for incident light which extends and is substantially uniform throughout the entire retroreflective area. In effect, the retroreflector ideally should reflect light approaching the retroreflector from all angles and possess this ability with little change for all portions of its retroreflective area.
It has previously been suggested to obtain wide angle response by tilting the axes of cube corners in two or more directions, either by dividing the retroreflector into sections or panels, each section having cube corners with one particular orientation differing from the cube corner orientation of the other sections; or by forming the light-reflecting units with a group of pins having individual cube axes pointing in different directions.
For example, U.S. Pat No. 3,954,324 to Arnott et al. illustrates the first-mentioned approach to solving the problem of a retroreflector which is retroreflective-responsive to incident light from widely differing incident angles. This patent discloses a retroreflector having a reflective wall of three portions, including a central portion and two side portions which project from opposite ends of the central portion. The latter has a plurality of conical reflector elements for reflecting incident light rays that are substantially parallel to a fore-and-aft axis of a housing for the reflective wall, such as a pavement marker, but which have relatively low optical efficiency for reflecting light rays making a substantial acute angle with respect to the fore-and-aft axis of the housing. The side portions of the reflective wall have reflector elements of a configuration other than conical and have less optical efficiency than the conical reflector elements for reflecting incident light rays that are in a horizontal plane parallel to the fore-and-aft axis but which have greater optical efficiency than the central portion for reflecting incident light rays that make a substantial acute angle with the plane of such fore-and-aft axis. Accordingly, while the three portions of the Arnott et al. reflective wall are collectively operable for incident light over a wide range of entrance angles, each of the three portions of the reflective wall is individually operative for only certain incident light for which the other two portions are substantially inoperative.
U.S. Pat. No. 3,926,402 to Heenan illustrates the second mentioned approach in which a set of pins is used to form a light-reflecting unit, the pins having individual axes pointing in different directions.